Passage of a mammalian cell through the cell cycle is regulated at a number of key control points. Among these are the points of entry into and exit from quiescence (G.sub.0), the restriction point, the G.sub.1 /S transition, and the G.sub.2 /M transition (for review. see Draetta (1990) Trends Biol. Sci. 15:378-383; and Sherr (1993) Cell 73:1059-1065). For a cell to pass through a control point and enter the next phase of the cell cycle, it must complete all of the events of the preceding cell cycle phase and, in addition, satisfy a number of check-point controls. Such controls act, for example, to ensure that DNA replication has been successfully completed before the onset of mitosis. Ultimately, information from these check-point controls is integrated through the regulated activity of a group of related kinases, the cyclin-dependent kinases (CDKs). Once a phase of the cell cycle has been successfully completed. phosphorylation of a critical substrates by activated CDKs allow passage of a cell cycle transition point and execution of the next cell cycle phase.
The ordered activation of the different CDKs constitutes the basic machinery of the cell cycle. The activity of CDKs is controlled bv several mechanisms that include stimulatory and inhibitory phosphorylation events, and complex formation with other proteins. To become active, CDKs require the association of a group of positive regulatory subunits known as cyclins (see, for example, Nigg (1993) Trends Cell Biol. 3:296). In particular, human CDK4 exclusively associates with the D-type cyclins (D1, D2, and D3) (Xiong et al. (1992) Cell 71:505; Xiong et al. (1993) Genes and Development 7:1572; and Matsushime et al. (1991) Cell 65:701) and, conversely, the predominant catalytic partner of the D-type cyclins is the CDK4 kinase (Xiong et al. (1992) Cell 71:505). The complexes formed bv CDK4 and the D-type cyclins have been strongly implicated in the control of cell proliferation during the G phase (Motokura et al. (1993) Biochem. Biophys. Acta. 1155:63-78; Sherr (1993) Cell 73:1059-1065; Matsushimi et al. (1992) Cell 71:323-334); and Kamb et al. (1994) Science 264:436-440).